Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by an electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid oxide fuel cell” (SOFC). Either pure hydrogen or reformate is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O2 molecule is split and reduced to two O−2 ions at the cathode/electrolyte interface. The oxygen ions diffuse through the electrolyte and combine the anode/electrolyte interface with four hydrogen ions to form two molecules water. The anode and the cathode are connected externally through the load to complete the circuit whereby four electrons are transferred from the anode to the cathode. When hydrogen is derived from “reformed” hydrocarbons, the “reformate” gas includes CO which is also converted to CO2 at the anode/electrolyte interface.
A single cell is capable of generating a relatively small voltage and wattage, typically about 0.7 volts and less than about 2 watts per cm2 of active area. Therefore, in practice it is usual to stack together in electrical series a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. Adjacent cells are connected electrically by “interconnect” elements in the stack, and the outer surfaces of the anodes and cathodes are electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam or a metallic mesh which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electrical terminals, or “current collectors,” connected across a load. For electrochemical reasons well known to those skilled in the art, an SOFC requires an elevated operating temperature, typically 750° C. or greater.
For steric reasons, fuel cells may be rectangular in plan view. Typically, gas flows into and out of the cells through a vertical manifold formed by aligned perforations near the edges of the components, the hydrogen flowing from its inlet manifold to its outlet manifold across the anodes in a first direction, and the oxygen flowing from its inlet manifold to its outlet manifold across the cathodes in a second direction.
An important limitation to improving power densities and increasing fuel utilization in a solid oxide fuel cell is localized fuel starvation in regions of the anode. The anode typically includes an active metal such as nickel (Ni). In local regions of the anode in which the concentration of hydrogen in the fuel being supplied is partially depleted, the partial pressure of O−2 can build up because the oxygen ion mobility is enabled by a completed circuit at each local segment of the overall cell and is independent of the hydrogen concentration present. O−2 which is not scavenged immediately by hydrogen or CO can attack and oxidize nickel in the anode.
What is needed is a means for preventing the formation of local areas of high oxygen ion concentration at the anode to protect the anode from corrosive attack.
It is a principal object of the present invention to prevent formation of a locally corrosive concentration of O−2 at the anode of a solid oxide fuel cell.
It is a further object of the present invention to improve the overall power output of a fuel cell stack.